The invention refers to a power wrench with a drive portion including a hydraulic cylinder, and a driven portion, the driven portion comprising a output shaft rotatably supported in a housing and rotated at intervals by a ratchet lever.
Hydraulic power wrenches are known that work based on the ratchet lever principle. When appropriately high hydraulic pressures are applied, such power wrenches can be used for very high torques. After a stroke of the hydraulic piston/cylinder unit, the output shaft tends to rotate backward by a small angle. This is due to the fact that the screw structure partly relaxes after each working stroke. To prevent such a relaxation, it is already known to have a blocking member engage the output shaft, the blocking member engaging an outer toothing of the output shaft, thereby preventing backward rotation. This entails the disadvantage of a safety risk caused by unreliable retaining systems. When the hydraulic unit that supplies pressure to the power screw driver is stopped, or in the event of a power breakdown or an incomplete stroke length, the apparatuses may come clear of the screw to be turned during the working process. This means a risk of accidents. Apparatuses with retaining latch systems may become twisted after the end moment has been reached and have to be detached tediously from the object to be screwed. The power wrench has to be brought up again to the maximum torque set and may lead to torque inaccuracies. After every stroke, the full clamping force is again applied to the apparatus and the screw bolt connection. This gives rise to high loads in the system and to constant bending stresses at the screw connection.